Actuators, such as hydraulic cylinders (also referred to as “hydraulic rams”), are used in a variety of applications to move or lift an object. Some of these are double-acting—meaning that they are designed to move a load in both the extend and retract directions. One particular application of this type of actuator is for the tilt and trim system for outboard marine engines. On many large outboard engines, two hydraulic rams perform the trim function (set the prop at a desired angle under thrust); while a separate cylinder performs the tilt function (tilt the prop out of water for storage and trailering). The trim rams have a limited stroke that defines the maximum angle that the engine can be set for high prop thrust, and must be capable of operating under high loads. In contrast, the tilt cylinder has a long stroke and typically operates under less load. It is preferred that the trim function occur rather slowly for optimum performance of the motor, while the tilt function occur relatively rapidly, so that the boat can be quickly inserted into or removed from the water. The trim and tilt cylinders can have separate hydraulic systems, and either a four-way valve or reversible pump can be used in each system to operate the cylinders in the extend and retract directions. As can be appreciated, the separate systems add cost, complexity and require significant space on the engine.
It is known to use a common pumping source (pump) for both the tilt cylinder and trim rams. This reduces the cost and complexity of the system somewhat, but as can be appreciated, still requires the cost and complexity of a tilt cylinder and separate trim rams. Since only a single pump is used, these systems also provide the extend and retract movements at the same speed, which can be undesirable. It is also known to use a single hydraulic cylinder and a common pumping source, which performs both the trim and tilt functions, with a first portion of the cylinder rod travel used for trim, and a further portion of the cylinder rod travel used for tilt. This further reduces the complexity of the system, but still provides only a single speed for both the trim and tilt functions.
On the other hand, it is known to provide a hi-low or staged pump for an actuator which operates at low flow, low speeds under high load; and at a high flow, high speeds under low loads. In one of these systems, shown in U.S. Pat. No. 3,903,698, a pair of conjointly-operated reversible pumps and a reversible motor provide a hi-low or staged hydraulic circuit during the extend movement of the actuator. Both the pumps provide high flow under low loads and low pressures; and only one of the pumps provides low flow under high loads and higher pressures. The output of the other pump is directed to tank through an unloader valve. The '698 system has some advantages over the prior systems in the extend mode, however, in the retract mode, the entire return flow from the actuator is directed to tank, and the cylinder is retracted—at one speed—by the flow provided by both pumps. As such, the actuator offers only a single retract stage, regardless of the load on the actuator, which again, can be undesirable in some applications.
Applicants therefor believe there is a demand for an improved hydraulic system for an actuator, particularly for marine applications which provides staged (hi-low) operation, in both the extend and retract modes of operation.